I, in the Shadows by Tori Bovalino

There’s a ghost haunting Drew Tarpin’s new room. Liam Orville has been dead for ten months and has no idea how to move on. But the longer he stays, the more likely it is he’ll degrade into an energy consuming husk―which Drew is more concerned about than her grades or her inability to make meaningful connections with other students. 

Drew is everything Liam never was when he was alive, but they do share some common ground: Drew finds herself hopelessly attracted to―and completely tongue-tied around―Hannah Sullivan, who happens to be Liam’s former best friend.

After a run-in with a ghost-eating monster leaves Drew and Liam desperate for answers, they strike up a deal: In return for Drew investigating why Liam is still around, he’ll help her talk to Hannah. But Liam’s time is running out, and if Drew doesn’t help him move on, he risks becoming a monster himself.